High Energy Heavy Ion PhysicsQuo Vadis ?

Rene BellwiedWayne State University(bellwied@physics.wayne.edu)

Link to cosmologyLink to cosmology

A QCD/QGP/RHIC primerA QCD/QGP/RHIC primer

The discovery of the sQGPThe discovery of the sQGP

Towards the most fundamental questionsTowards the most fundamental questions

Motivation for Relativistic Heavy Ion Collisions

Two big connections: cosmology and QCD

Going back in time…

Age Energy Matter in universe 0 1019 GeV grand unified theory of all forces

10-35 s 1014 GeV 1st phase transition

(strong: q,g + electroweak: g, l,n)

10-10 s 102 GeV 2nd phase transition(strong: q,g + electro: g + weak: l,n)

10-5 s 0.2 GeV 3rd phase transition(strong:hadrons + electro:g + weak: l,n)

3 min. 0.1 MeV nuclei

6*105 years 0.3 eV atoms

Now 3*10-4 eV = 3 K(15 billion years)

RHIC, LHC & FAIR

RIA & FAIR

Evolution of Forces in Nature

Connection to Cosmology• Baryogenesis ? Separation of Matter and Antimatter – can it

happen at the phase transition ?

• Dark Matter Formation ? – can it happen at the phase transition ?

• Dark Energy Formation – can it happen at the phase transition ?

• Is matter generation in cosmic medium (plasma) different than matter generation in vacuum ?

• Can fluctuations at the phase transition explain an anisotropic matter distribution in the universe ?

Sakharov (1967) – three conditions for baryogenesis

• Baryon number violation• C- and CP-symmetry violation• Interactions out of thermal equilibrium

• Currently, there is no experimental evidence of particle interactions where the conservation of baryon number is broken: all observed particle reactions have equal baryon number before and after. Mathematically, the commutator of the baryon number quantum operator with the Standard Model Hamiltonian is zero: [B,H] = BH - HB = 0. This suggests physics beyond the Standard Model

• The second condition — violation of CP-symmetry — was discovered in 1964 (direct CP-violation, that is violation of CP-symmetry in a decay process, was discovered later, in 1999). If CPT-symmetry is assumed, violation of CP-symmetry demands violation of time inversion symmetry, or T-symmetry. Under investigation

• The last condition states that the rate of a reaction which generates baryon-asymmetry must be less than the rate of expansion of the universe. In this situation the particles and their corresponding antiparticles do not achieve thermal equilibrium due to rapid expansion decreasing the occurrence of pair-annihilation.

A mass problem of universal proportion

• The stars and gas in most galaxies move much quicker than expected from the luminosity of the galaxies.

• In spiral galaxies, the rotation curve remains at about the same value at great distances from the center (it is said to be ``flat'').

• This means that the enclosed mass continues to increase even though the amount of visible, luminous matter falls off at large distances from the center.

Something else must be adding to the gravity of the galaxies Something else must be adding to the gravity of the galaxies without shining. We call it without shining. We call it Dark MatterDark Matter ! ! According to measurements it accounts for > 80% of the mass According to measurements it accounts for > 80% of the mass in the universe.in the universe.

The cosmic connection of RHI physics

Witten’s ‘Cosmic Separation of phases’Witten’s ‘Cosmic Separation of phases’ (Phys.Rev.D 30 (1984) 272)(Phys.Rev.D 30 (1984) 272)

basic parameter: massbasic parameter: mass

Originally: strange quark matter was a prime candidate for Originally: strange quark matter was a prime candidate for dark matter (as recent as SQM 2003)dark matter (as recent as SQM 2003)

Dark Matter vs. Luminous Matter distributionBullet Cluster, 3.4 Billion Lightyears from Earth

X-ray image vs. gravitational lensing

The universe is accelerating….

Based on supernovae measurements

You need DARK ENERGY as an explanation (!?)

Dark Energy does not kick in at the time of the Big Bang !

The cosmic connection of RHI physics

Let’s understand mass generation in the luminous matter

What do we know about quark masses ?

Why are quark current masses so different ?

There is no answer to this questions.There likely will be no answer to this question !

Nature’s constants:-speed of light, electric charge, quark current masses (?)

Very little is known, very little can be explained

Standard model is symmetricAll degrees of freedom are massless

Electro-weak symmetry breakingvia Higgs field (m of W, Z, Mechanism to generate current quarkmasses(but does not explain their magnitude)

Chiral symmetry breakingvia dynamical quarksMechanism to generate constituentquark masses(but does not explain hadronization)

We can’t answer the question of massgeneration at the most fundamental level,but can we answer the question of mass

generation at the nuclear level ?

The fundamental problem: how is baryonic mass generatedBased on quark interactions (5+10+10 = 935 MeV/c2) ?

Theory:QuantumChromoDynamics

Theoretical and computational (lattice) QCDIn vacuum: - asymptotically free quarks have current mass- confined quarks have constituent mass- baryonic mass is sum of valence quark constituent masses

Masses can be computed as a function of the evolving coupling strength or the ‘level of asymptotic freedom’, i.e. dynamic masses.

But the universe was not a vacuum at the time of hadronization,it was likely a plasma of quarks and gluons. Is the mass generationmechanism the same ?

The main features of Quantum Chromodynamics (QCD)

• Confinement– At large distances the effective coupling between quarks is large, resulting in

confinement.– Free quarks are not observed in nature.

• Asymptotic freedom– At short distances the effective coupling between quarks decreases

logarithmically.– Under such conditions quarks and gluons appear to be quasi-free.

• (Hidden) chiral symmetry– Connected with the quark masses– When confined quarks have a large dynamical mass - constituent mass– In the small coupling limit (some) quarks have small mass - current mass

Strong color fieldForce grows with separation !!!

Analogies and differences between QED and QCDto study structure of an atom…

“white” proton

…separate constituents

Imagine our understanding of atoms or QED if we could not isolate charged objects!!

nucleus

electron

quark

quark-antiquark paircreated from vacuum

“white” proton(confined quarks)

“white” 0

(confined quarks)

Confinement: fundamental & crucial (but not understood!) feature of strong force- colored objects (quarks) have energy in normal vacuum

neutral atom

To understand the strong force and the phenomenon of confinement:Create and study a system of deconfined colored quarks (and gluons)

A mechanism of hadronization in vacuum:String Fragmentation

High momentum current mass quark pair forms flux tube in a collision = string of energy (string tension) i.e. dynamical quark field which fragments into hadrons when string tension becomes too large.

Describes e+e- and p-pbar and p-p collisions well.

Hadronization in medium (i.e. during universe expansion) could be different because medium might affect the mechanism.

The temperature dependent running coupling constant s and its effect on

mass generation above TcO.Kaczmarek et al. (thermal mass, LQCD) (hep-lat/0406036)

1.05 Tc

1.5 Tc

3 Tc

6 Tc12 Tc

in an expanding system: interplay betweendistance and temperature

Massive partons above Tce.g. P.Levai and U.Heinz (hep-ph/9710463)

Lattice QCD:Chiral Symmetry is restored at Tc

One goal: Proving asymptotic freedom in the laboratory.

• Measure deconfinement and chiral symmetry restoration under the conditions of maximum particle or energy density.

D. GrossH.D. PolitzerF. Wilczek

QCD Asymptotic Freedom (1973)

Nobel Prize 2005

Before QCD

Density of hadron mass states dN/dM increases exponentially with mass.

dN

dM~ exp M

TH

Broniowski, et.al. 2004TH ~ 21012 oK

Rolf Hagedorn GermanHadron bootstrap model and limiting temperature (1965)

Energy diverges as T --> TH

Maximum achievable temperature?

“…a veil, obscuring our view of the very beginning.” Steven Weinberg, The First Three Minutes (1977)

Karsch, Redlich, Tawfik, Eur.Phys.J.C29:549-556,2003

/T4

g*S

“In 1972 the early universe seemed hopelessly opaque…conditions of ultrahigh temperatures…produce a theoretically intractable mess. But asymptotic freedom renders ultrahigh temperatures friendly…” Frank Wilczek, Nobel Lecture (RMP 05)

QCD to the rescue!

Replace Hadrons (messy and numerous)

by Quarks and Gluons (simple

and few)

Ha

dro

n g

as

Thermal QCD ”QGP” (Lattice)

Nobel prize for Physics 2005

Kolb & Turner, “The Early Universe”

QC

D T

rans

ition

e+e- A

nnih

ilatio

n

Nuc

leos

ynth

esis

D

ecou

plin

g

Mes

ons

free

ze o

ut

Hea

vy q

uark

s an

d bo

sons

free

ze o

ut

“Before [QCD] we could not go back further than 200,000 years after the Big Bang. Today…since QCD simplifies at high energy, we can extrapolate to very early times when nucleons melted…to form a quark-gluon plasma.” David Gross, Nobel Lecture (RMP 05)

Thermal QCD -- i.e. quarks and

gluons -- makes the very early universe tractable; but where is the experimental

proof?

g*S

Generating a deconfined state

Nuclear Matter(confined)

Hadronic Matter(confined)

Quark Gluon Plasmadeconfined !

Present understanding of Quantum Chromodynamics (QCD)• heating• compression deconfined color matter !

Expectations from Lattice QCD/T4 ~ # degrees of freedom

confined:few d.o.f.

deconfined:many d.o.f.

TC ≈ 173 MeV ≈ 21012 K ≈ 130,000T[Sun’s core]C 0.7 GeV/fm3

Suggested Reading

• October 2006 issue of Nature:“Did the Big Bang Boil ? ” by F. Wilczek

• …the answer as far as the quark-hadron transition is concerned is ‘No’. QCD evolves smoothly with temperature there is no thermodynamic phase transition.

• Heavy Ion collisions at RHIC and the LHC can produce fireballs with a significant excess of baryons over anti-baryons, or different effective temperatures for quarks and gluons – possibilities that did not occur in the cosmic Big Bang. In those new circumstances do true phase transitions occur ?

A phase transition into what ?• With the liquid-gas phase transition established (ground state liquid drop nuclei transition to a hadron gas) the question was:

What comes next ? A weakly interacting plasma.• Edward Shuryak (1971) : name it the Quark Gluon Plasma

Cabibo-Parisi, PLB59 (1975) G.Baym, NSAC-LRP (1983)

The phase diagram of QCDT

em

per

atu

re

baryon density

Neutron stars

Early universe

nucleinucleon gas

hadron gascolour

superconductor

quark-gluon plasmaTc

0

critical point ?

vacuum

CFL

Study all phases of a heavy ion collision

If the QGP was formed, it will only live for 10-22 s !!!!BUT does matter come out of this phase the same way it went in ???

microexplosions femtoexplosions

s 0.1 J 1 J

1017 J/m3 5 GeV/fm3 = 1036 J/m3

T 106 K 200 MeV = 1012 K

rate 1018 K/s 1035 K/s

Energy density of matter

high energy density: > 1011 J/m3

P > 1 MbarI > 3 X 1015W/cm2 Fields > 500 Tesla

QGP energy density > 1 GeV/fm3

i.e. > 1030 J/cm3

Step 1: Measuring a reference systemIn order to prove that we form a phase of matter that

behaves different than the vacuum we need to understand our results in pp collisions ?

hadrons

hadrons

leading particle

Jet: A localized collection of hadrons which come from a fragmenting parton

Parton Distribution Functions

Hard-scattering cross-section

Fragmentation Function

a

b

c

dParton Distribution FunctionsHard-scattering cross-sectionFragmentation Function

c

chbbaa

abcdba

T

hpp

z

Dcdab

td

dQxfQxfdxdxK

pdyd

d

0

/222

)(ˆ

),(),(

High pT (> 2.0 GeV/c) hadron production in pp collisions:

~

Hadronization in QCD (the factorization theorem)

“Collinear factorization”

0 in pp: well described by NLO (& LO)

• Ingredients (via KKP or Kretzer)– pQCD– Parton distribution functions– Fragmentation functions

• ..or simply PYTHIA…

p+p->0 + X

Hard

Scattering

Thermally-shaped Soft Production

hep-ex/0305013 S.S. Adler et al.

“Well Calibrated”

pp at RHICStrangeness formation in QCD

nucl-ex/0607033

How strong are the NLO correctionsin LO calculations (PYTHIA) ?

• K.Eskola et al.(NPA 713 (2003)):Large NLO corrections notunreasonable atRHIC energies.

Should be negligibleat LHC (5.5 or 14 TeV).

STAR

LHC

New NLO calculation based on STAR data (AKK, hep-ph/0502188, Nucl.Phys.B734 (2006))

K0s

apparent Einc dependence of separated quark contributions.

Mt scaling in pp

Breakdown of mT scaling in pp ?

mT slopes from PYTHIA 6.3

Gluon dominance at RHICPYTHIA: Di-quark structures in baryon production cause mt-shiftRecombination: 2 vs 3 quark structure causes mt shift

Collision Energy dependence of baryon/meson ratio - baryon production in pp is simply not well understood

Ratio vs pT seems very energy dependent (RHIC < < SPS or FNAL), LHC ?

Not described by fragmentation !(PYTHIA ratios at RHIC and FNAL are equal)

Additional increase with system size in AA

Both effects (energy and system size dependence) well described by recombination

Conclusions for RHIC pp data

• We are mapping out fragmentation and hadronization in vacuum as a function of flavor.

• What we have learned:– Strong NLO contribution to fragmentation even for light quarks at RHIC

energies

– Quark separation in fragmentation function very important. Significant non-valence quarks contribution in particular to baryon production.

– Gluon dominance at RHIC energies measured through breakdown of mt-scaling and baryon/meson ratio. Unexpected small effect on baryon/antibaryon ratio

– Is there a way to distinguish between fragmentation and recombination ? Does it

matter ?

• What will happen at the LHC ? What has happened in AA collisions (hadronization in matter) ?

The future: unprecedented physics reach at LHC (ALICE – pp)

(charged particle spectra)

enormous reach in multiplicity and transverse momentum.Could this system behave collectively ??

Step 2: Proving the existence of a new phase of matterCan we prove that we have a phase that

behaves different than elementary pp collisions ?

Three steps:

a.) prove that the phase is partonic

b.) prove that the phase is collective

c.) prove that the phase characteristics are different from the QCD vacuum

Fate of jets in heavy ion collisions?

p

p

?

Au+Au

idea: p+p collisions @ same sNN = 200 GeV as reference

?: what happens in Au+Au to jets which pass through medium?

Prediction: scattered quarks radiate energy (~ GeV/fm) in the colored medium: “quenches” high pT particles “kills” jet partner on other side

Major discoveries in AuAu collisions

‘The Big Three’(leading to the discovery of the sQGP

= the Perfect Quark Gluon Liquid = AIP Science Story of 2005)

STAR, nucl-ex/0305015

energyloss

pQCD + Shadowing + Cronin

pQCD + Shadowing + Cronin + Energy Loss

# I: The medium is dense and partonic

Deduced initial gluon density at = 0.2 fm/c dNglue/dy ≈ 800-1200

≈ 15 GeV/fm3, eloss = 15*cold nuclear matter

(compared to HERMES eA or RHIC dA) (e.g. X.N. Wang nucl-th/0307036)

An important detail: the medium might not be totally opaqueThere are specific differences to the flavor of the probe

Theory: there are two types of e-loss:radiative and collisional, plus dead-cone effect for heavy quarksFlavor dependencies map out the process of in-medium modification

Experiment: there arebaryon/meson differences

# II: The medium behaves like a liquid

x

yz

Strong collective flow:elliptic and radial expansion withmass ordering

requires partonic hydrodynamics:strong coupling,small mean free path,lots of interactionsNOT plasma-like more like a perfect liquid (near zero viscosity, d.o.f. ?)

# III: The medium consists of constituent quarks ?

baryonsbaryons

mesonsmesons

Recombination vs. Fragmentation(a different hadronization mechanism in medium than in vacuum ?)

Recombination at moderate PT

Parton pt shifts to higher

hadron pt.

Fragmentation at high PT:

Parton pt shifts to lower

hadron pT

recombining partons:p1+p2=ph

fragmenting parton:ph = z p, z<1

Recomb.

Frag.

liquid ?

liquid

plasma

gas

Hirano, Gyulassy (2006)

Consequences of a perfect liquid• All “realistic” hydrodynamic calculations for RHIC

fluids to date have assumed zero viscosity

= 0 “perfect fluid”– But there is a (conjectured) quantum limit:

– Where do “ordinary” fluids sit wrt this limit?

– RHIC “fluid” mightbe at ~2-3 on this scale (!)

sDensityEntropy

4

)(4

T=10T=101212 KK

Description might require new dimensions• Expanding our theoretical tools – the Maldacena conjecture

– AdS/CFT for calculating static and dynamic properties of strongly-coupled gauge theories

• There is a string dual to AdS/CFT: 4 dim. SUSY Yang-Mills

• Determine viscosity and entropy density in RHIC by calculating it in a 10-dim black hole calculation

Color Screening

cc

MULTIPLICITY

Entropy Black Hole Area

DISSIPATION

Viscosity Graviton

Absorption

Explaining the Connection

Maldacena’s

conjecture

3) Strongly 3) Strongly Coupled Coupled

(Conforma(Conformal) gauge l) gauge

Field Field Theories Theories

(CFT)(CFT)

1) Weakly Coupled 1) Weakly Coupled (classical) gravity in (classical) gravity in Anti-deSitter Space Anti-deSitter Space

(AdS)(AdS) 2) 2)

Suggested Reading• November, 2005 issue of Scientific

American“The Illusion of Gravity” by J. Maldacena

• A test of this prediction comes from the Relativistic Heavy Ion Collider (RHIC) at BrookhavenNational Laboratory, which has been colliding gold nuclei at very high energies. A preliminary analysis of these experiments indicates the collisions are creating a fluid with very low viscosity. Even though Son and his co-workers studied a simplified version of chromodynamics, they seem to have come up with a property that is shared by the real world. Does this mean that RHIC is creating small five-dimensional black holes? It is really too early to tell, both experimentally and theoretically. (Even if so, there is nothing to fear from these tiny black holes-they evaporate almost as fast as they are formed, and they "live" in five dimensions, not in our own four-dimensional world.)

In the past six months: >50 preprints on AdS/CFT !• “The stress tensor of a quark moving through N=4

thermal plasma”, J.J. Friess et al., hep-th/0607022

Our 4-d Our 4-d worlworl

dd

String String theorist’theorist’

s 5-d s 5-d worldworld

The stuff The stuff formerly formerly known as QGPknown as QGP

Heavy Heavy quark quark

moving moving through through

the the mediummedium

Energy loss Energy loss from from string string dragdrag

Jet Jet modificationmodifications from wake s from wake

fieldfield

An explosion of new papers based on string duality to AdS/CFT

• J.Friess et al., hep-th/0607022Stress tensor of a quark moving through a N=4 thermal plasma

• J.Friess et al., hep-th/0605292Dissipation from a heavy quark moving through a N=4 super Yang-Mills plasma

• Liu, Rajagopal, Wiedemann, hep-th/0607062An AdS/CFT calculation of screening in a hot wind

• Liu, Rajagopal, Wiedemann, hep-th/0605178Calculating the jet quenching parameter from AdS/CFT

SU(3) gauge theory (2+1) flavor QCD

Resummed perturbative calculations from :Blaizot, Iancu, Rebhan, hep-ph/0303185

Lattice data on pressure and entropy density at high temperatures can be described by re-summed perturbation theory. At high T deviation from SB limit only 10%

The perfect liquid, when does it vaporize ?

Comparison with re-summed perturbation theory, effective 3d theory and additional lattice data on quark number susceptibility and the Debye mass suggest thatwe have wQGP for T > 2 Tc

Quo Vadis ?Many very important issues are left to investigate, e.g.

a.) need evidence for chiral symmetry restoration

b.) what are the initial conditions ?

c.) what is the hadronization mechanism in medium and in vacuum ? How are hadronic masses generated ?

d.) is there a critical point at finite net baryon density and howdo the features of the phases change close to it ?

e.) is there a Color Glass Condensate at very low x ?

Chirality: Why Resonances ?

2212

21 ppEEminv

Bubble chamber, BerkeleyM. Alston (L.W. Alvarez) et al., Phys. Rev. Lett. 6 (1961) 300.

Invariant mass (K0+) [MeV/c2]

K*-(892)

640 680 720 760 800 840 880 920

Nu

mb

er

of

even

ts

0

2

4

6

8

10

Luis Walter Alvarez 1968 Nobel Prize for

“ resonance particles ” discovered 1960

K* from K-+p collision system Kp p

K

Resonances are:

• Excited state of a ground state particle.• With higher mass but same quark content.• Decay strongly short life time (~10-23 seconds = few fm/c ), width = natural spread in energy: = h/t. Breit-Wigner shape

• Broad states with finite and t, which can be formed by collisions between the particles into which they decay.

Why Resonances?:• Surrounding nuclear medium may change resonance properties• Chiral symmetry breaking: Dropping mass -> width, branching ratio

Strange resonances in medium

Short life time [fm/c] K* < *< (1520) < 4 < 6 < 13 < 40

Red: before chemical freeze outBlue: after chemical freeze out

Medium effects on resonance and their decay products before (inelastic) and after chemical freeze out (elastic).

Rescattering vs. Regeneration ?

Resonance Signal in Au+Au collisions

STAR Preliminary

Au+Au minimum biaspT 0.2

GeV/c

|y| 0.5

K*0 + K*0

(1520)

STAR Preliminary

(1020)

STAR Preliminary

*± +*±

K(892)

Chiral Symmetry Restoration

Ralf Rapp (Texas A&M) J.Phys. G31 (2005) S217-S230

Vacuum At Tc: Chiral Restoration

Measure chiral partnersNear critical temperature Tc (e.g. and a1)

Data: ALEPH Collaboration R. Barate et al. Eur. Phys. J. C4 409 (1998)

a1 +

Lattice QCD predicts a critical point (critical parameters at finite baryon density)

Critical Endpoint in Effective Models

Compilation by Stephanov

Do we need a color glass condensate to explain experimental puzzle in HEP ?

Froissart Bound

The Gluon ‘blows up’

RHIC

The gluon ‘blows up’

Gluon saturation

Gluon density in hadrons

Motivation for the CGC

A time evolution for ‘matter’

Final phase diagram

The reach of RHIC

CGC at LHC

What are the initial conditions ? Color Glass Condensate (CGC): gluon saturation at low Q2.

Measure – Mid – forward rapidity

correlations (hep-ph/0403271)– Direct photons at forward

rapidities – HBT (coherence of sea-quark

source?)– Drell-Yan in forward region (hep-

ph/040321)– RpA, RAA of heavy mesons in

forward direction (hep-ph/0310358)

requires tracking, calorimetry and PID over large -range.

ln (

1/x)

Onium physics – the complete program – Melting of quarkonium states (Deconfinement TC)

Tdiss(’) < Tdiss((3S)) < Tdiss(J/) Tdiss((2S)) < Tdiss((1S))

Color screening of heavy flavor will tell us theInitial temperature and its evolution with time !

The initial thermal conditions

Requirements for a complete onium program• Full coverage high resolution forward calorimetry in order to

measure not only the J/ but also the c and the Y States

Full coverage (||< 3) calorimetry and muon absorbers give us up to 1,000,000 c, 10,000 Y(2s) and 10,000 Y(3s) per RHIC year. The J/ alone is not sufficient !

There is plenty to do…

• ..and all of it is exciting• ..and all of it is fundamental• ..and all of it will benefit the understanding of

QCD, the standard model, and potentially new physics

• ..and all of it will shed light on the evolution of the universe

• ..and we might understand the generation of mass, one of the most fundamental principles in nature